This invention relates to methods for obtaining a high-accuracy attitude sensor for a spin stabilized satellite.
Some satellites are attitude stabilized by causing such satellites to spin around a predetermined axis. Such satellite usually employ optical or infrared sensors to determine the orientation of the satellite's spin axis with respect to celestial objects such as the earth, moon and sun. One frequently used is an infrared horizon crossing indicator (HCI) which views a narrow field directed outward from the spacecraft at some angle with the spin axis of the spacecraft. The horizon crossing indicator scans a circle in space around the spin axis by the rotation of the satellite on its spin axis and produces pulse signals in the field of the infrared detector of the HCI crosses the earth's horizon. The phase and interval between crossings provides the attitude information.
The direction of the sun or moon can also be found with such sensors, but the angular subtense of such celestial objects will generally be so small that in using a fixed angle the scan circle may not intersect such objects. This problem can be overcome by incrementing the angle with respect to the spin axis by the diameter of the field of view of the HCI after each rotation. However, this greatly increases the acquisition time and, in addition, the HCI could lose the object during rapid maneuvers.